Disc brake rotor direction question?

uart
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Disc brake rotor direction question?

Postby uart » Sat Sep 09, 2017 10:14 am

I'm just fixing up an old folding bike with front disc brakes. I think I need a new rotor (this one is a bit warped) but I'm going to at least have a go at straightening it first.

Being unfamiliar with bike disc brakes (all my other bikes have rim brakes) I initially thought that the rotor was on backwards. The direction of the disc rotation seemed wrong, but now I've seen some others I find that it was actually correct.

So my question is why do they choose the direction of rotation such that it puts the spokes in compression rather than tension? I just seems wrong.

See for example this image which is similar to the one I have. See that the braking force in the radial "spoke" sections of the rotor will be pushing rather than pulling. Has anyone else ever noticed that and thought it weird?

Image
Last edited by uart on Sat Sep 09, 2017 1:45 pm, edited 1 time in total.

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Duck!
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Re: Disc brake rotor direction question?

Postby Duck! » Sat Sep 09, 2017 10:59 am

The direction of the rotor spokes supports the braking part of the rotor and keeps it in the right place under heavy brake load. If the rotor is reversed so the spokes angle backward, the rotor will collapse.
I had a thought, but it got run over as it crossed my mind.

uart
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Location: Newcastle

Re: Disc brake rotor direction question?

Postby uart » Sat Sep 09, 2017 1:49 pm

Thanks Duck. So it's really the hoop strength of the outer "rim" part of the rotor that is supporting it. The pushing "spokes" also push the outer "rim" into tension. Yeah I guess that makes sense. :)

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Nate
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Re: Disc brake rotor direction question?

Postby Nate » Mon Sep 11, 2017 1:37 pm

i thought the same thing, & you're correct - steel is always in tension, NOT compression.

dont focus on the arms being in compression or tension, look at the disc.

in the pic above the arms are in compression, BUT the rotor disc is actually in tension, i.e. you're trying to stretch the disc & make it bigger.
If you had the arms in tension, you'd have the disc in compression & trying to make the disc smaller - which would/could fail & distort.

Thats my reasoning IMHO... feedback from anyone else welcomed!

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